The present invention relates to a power steering system for assisting a wheel steering force. More particularly, it relates to a control system for an electric motor which can exhibit an effect on steering-feel improvements (i.e., assist-force enhancement and micro-vibration suppression) in a steering wheel.
As a power assist system for assisting the wheel steering force in response to input of a steering torque from driver, a system which uses a hydraulic mechanism is of ordinary type. As an example of this conventional technology, there exists the following one: Namely, a hydraulic pump is driven by an electric motor so as to generate a hydraulic pressure. Then, the hydraulic pressure thus generated is controlled by feedback of the steering torque inputted from the steering wheel, thereby generating a steering assist force. In the conventional technology, the steering torque is detected from the value of a torque sensor, and in order to generate the steering assist force corresponding to the torque, pressure supplied to the hydraulic cylinder is controlled. In the conventional technology, regardless of the steering state, the electric motor is always controlled so that the motor torque will become proportional to a motor current command.
In JP-A-2003-212141 (pp. 2 to 5, FIG. 1), a power steering system is disclosed which includes the following configuration components: A steering axis interconnected to a steering mechanism, a hydraulic power cylinder for assisting a steering force to the steering mechanism, a reversible pump equipped with a pair of ejection apertures for supplying a hydraulic pressure to both-sides hydraulic chambers of the hydraulic power cylinder via a first channel and a second channel, an electric motor for driving the reversible pump to perform a normal/reverse rotation driving, a steering-direction detection device for detecting rotation direction of the steering axis, a steering-force detection device for detecting the steering force applied to the steering axis, an electric-motor control device for calculating a driving current outputted to the electric motor in order to drive the hydraulic power cylinder to generate a desired hydraulic pressure on the basis of a steering-force signal detected by the steering-force detection device and a steering-axis rotation-direction signal detected by the steering-direction detection device, a temperature measurement device for measuring temperature of the electric motor and/or an electric-motor driving rotation element, and an overheat protection device for lowering upper-limit value of a driving current outputted from the electric-motor control device by a temperature rise in the electric motor and/or the electric-motor driving rotation element measured by the temperature measurement device. In this power steering system, there are further provided a returning detection device for detecting return of the steering axis, and an overheat-protection release device which, when the returning of the steering axis has been detected by the returning detection device, allows temporary output of a driving current that exceeds the upper-limit value of the driving current lowered by the overheat protection device.
Also, an electric power steering system for assisting the steering with the use of an electric motor alone, i.e., without the use of the hydraulic mechanism, starts to prevail in its main use among small-sized cars. In this technology, at a steering-switching time from a steering operation to a holding operation, or from the holding operation to the steering operation, an assist force of the electric motor is adjusted. For example, in JP-A-8-295257 (pp. 2 to 5, FIG. 2), an electric power steering system is disclosed which includes the following configuration components: A vehicle-velocity detection device for detecting vehicle velocity, a steering-torque detection device for detecting steering torque of a steering wheel, an electric motor for assisting a steering force of the steering wheel, a control device for controlling the electric motor in correspondence with the detected vehicle velocity and the detected steering torque, a steering-change detection device for detecting the switching between the steering operation state where the steering wheel has a steering velocity and the holding operation state where the steering velocity is equal to zero, and an increase/decrease device for performing a current decrease down to a predetermined value with respect to the electric motor if the switching from the steering operation state to the holding operation state has been detected in a state where the detected vehicle velocity is lower than a set vehicle velocity, and for performing in a retarded manner a current increase up to a steering operation value with respect to the electric motor if the switching from the holding operation state to the steering operation state has been detected.